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Genetic diversity and population genetic structure in the South African commercially important shark species, the common smoothhound (Mustelus mustelus)

Maduna, Simo Njabulo (2014-12)

Thesis (MSc)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: Deciphering patterns of intraspecies population genetic structuring in commercially
important shark species is essential for an integrated fisheries management approach to
conservation of regional biodiversity. The common smoothhound shark, Mustelus mustelus,
is an overexploited, commercially and recreationally important shark species in South Africa.
Considering the vulnerable status of the common smoothhound shark and due to very limited
available genetic information, this study aimed to develop molecular markers, assess patterns
of genetic diversity and population connectivity along the South African coast using
multilocus data generated from 12 microsatellite markers and the mitochondrial gene, NADH
dehydrogenase subunit 4 (ND4). The cross-species amplification of microsatellites proved
useful for genetic diversity and population genetic analysis of the common smoothhound
shark. These microsatellites could aid in the molecular characterisation of other endemic and
cosmopolitan species and provide valuable tools for the conservation of potentially
threatened or exploited shark species. For the microsatellite data, moderate levels of genetic
diversity based on the heterozygosity, allelic richness and haplotype diversity were found in a
total of 144 individuals sampled across eight study populations. Estimates for pairwise
population differentiation, F-statistics, AMOVA and factorial correspondence analysis (FCA)
indicated significant genetic structure within and between west- and east coast populations.
Additionally, Bayesian clustering analyses detected two putative ancestral gene pools,
supporting the presence of a biogeographic barrier at the Cape Agulhas region and therefore
genetic discontinuity between the Indian and Atlantic Ocean samples. On the contrary,
mitochondrial data indicated that common smoothhound shark is genetically homogenous with substantial interoceanic gene flow. Such conflicting signals found between nuclear and
mitochondrial DNA (mitonuclear discordance) can be attributed to a number of factors and
could simply be due to the inherent differences in marker properties or an indication of sex
biased dispersal. Despite an indication of an expanding common smoothhound shark
population based on both marker types, a contemporary genetic bottleneck may have gone
undetected as genetic divergence was very low in some of the study populations.
Nonetheless, contemporary restriction to gene flow and historical demographics such as
range expansion are proposed as the most likely forces explaining genetic structure in
present-day common smoothhound sharks in South Africa. For future sustainable exploitation of common smoothhound shark, the possible existence of two genetically differentiated
populations and observed asymmetric gene flow along the South African coast should be
taken into consideration. It is also recommended that in the future further evaluations of finescale
genetic structure and seasonal migration patterns in this commercially important species
are conducted in order to allow integration of this knowledge into existing fisheries
management practices.